The manufacture of flexible polyurethane foams is well known, its chemistry and technology being discussed in reference books such as "Polyurethane Chemistry & Technology", H. Saunders and K. C. Frisch, Robert E. Krieger Publ. Co. (1983); "Polyurethane Handbook", G. Oertel, Hansen Verlag, Munich (1985); and "The ICI Polyurethanes Book", G. Woods, Wiley & Sons (1990). The preparation of flexible foams by reacting a polyisocyanate and a polyol in the presence of a blowing agent has been described in, e.g., U.S. Pat. Nos. 5,114,989; 4,822,517; 4,256,849; and 5,091,437; European Patent Application Publication Nos. 111121; 022617; 010850 and 422471 and United Kingdom Patent No. 1422056; and 1479658. The techniques described within these reference books have been used commercially to produce flexible polyurethane foams for various applications, such as furniture, bedding, automobiles, etc.
Flexible polyurethane foams are generally produced by reacting polyols and polyisocyanates in the presence of water used as a carbon dioxide generator. Halocarbons, such as chlorofluorocarbons ("CFC"'s) have been used as auxiliary blowing agents to supplement the blowing action of the water. Toluene diisocyanate (TDI) based systems have been found to be useful as the polyisocyanate component. More recently, diphenylmethane diisocyanate (MDI) based systems have also found application in the manufacture of flexible polyurethane foams. The MDI-based systems are advantageous because of their lower volatility in comparison to TDI. However, it has been found difficult to produce flexible foams utilizing MDI having sufficiently low density which is comparable to TDI-based systems.
The manufacture of flexible polyurethane foams from isocyanate-containing prepolymers utilizing a mixture of MDI and polymeric forms of MDI is disclosed e.g., in U.S. Pat. No. 4,365,025. This patent discloses flexible polyurethane foams having low compression set values prepared by reacting an aqueous phase with a resin phase containing a prepolymer derived from MDI and a polyol having at least 50% by weight of oxyethylene groups. U.S. Pat. No. 5,114,989 is directed to isocyanate-terminated prepolymers prepared from the reaction of a polyisocyanate comprising 4,4'-diphenylmethane diisocyanate with a polyoxyalkylene polyol having an oxyethylene content of from 40-68% by weight. The prepolymers of this patent are disclosed as being useful when mixed with an active hydrogen-containing composition in preparing flexible polyurethane foams. U.S. Pat. No. 4,668,708 discloses a process for the preparation of elastic polyurethane flexible foams comprising reacting an organic polyisocyanate prepolymer having an isocyanate content of 6-20% by weight prepared by reacting TDI and/or MDI with a di- to tetra-functional polyether polyol having an oxyethylene terminal group content of at least 10-30% by weight with a mixture of compounds containing reactive hydrogen atoms, a blowing agent, a catalyst and optionally auxiliaries and/or additives. The compounds containing the reactive hydrogen atoms include from 10-25 parts by weight water. U.S. Pat. No. 5,070,114 discloses polymethane prepolymer compositions obtained by reacting a polyol component comprising at least one polyoxyalkylene polyol having an equivalent weight of from 500 to 5000 and an average oxyethylene content of 5 to 30% and (b) a stoichiometric excess of an MDI composition.
The preparation of flexible polyurethane foams is also generally described in U.S. Pat. Nos. 5,122,548; 4,910,231; 4,916,168; 5,043,360 and European Patent Application Publication Nos. 257441 and 393903.
However, none of the references discussed above disclose reaction systems or processes which obtain MDI-based flexible foams having a density which is low enough to be comparable with TDI-based foams. Thus, processes and systems for the production of such low density MDI-based foams are needed.
It is thus an object of the present invention to provide reaction systems comprising MDI and processes utilizing such reaction systems which will provide flexible polyurethane foams having a low density which compares favorably to TDI-based systems. It is a further object of the invention to provide flexible polyurethane foams having significantly improved physical properties, such as a lower compression value and a higher tear strength, in spite of their lower density in comparison to conventional MDI-based foams.